A tube fitting commonly used in the prior art is one wherein a nut is slipped over the end of the tubing and then a sleeve with two conically tapered ends is slipped over the end of the tube. The tube end is then inserted in a second coupling member, which has an annular lateral shoulder therein at about a 45-degree angle to the tube axis. The nut or first fitting member also has about a 45-degree angle on an annular lateral shoulder, and as the two fitting members are threaded together, these two 45-degree lateral shoulders axially compress the conical sleeve. Because of the angle of only about 20 degrees on the sleeve, this sleeve is crushed radially inwardly to seal against the outer surface of the tube and the outer conical surface seals with the annular shoulder in the second fitting member. This type of fitting has been used for many years, as one example, on a gasoline line to an automobile carburetor.
A second prior art fitting utilized a flared end on the tubing, as exemplified by U.S. Pat. No. 3,195,936.
A third form of prior art is shown in U.S. Pat. No. 3,765,708 with a pair of annular beads at the end of the tube. However, these are caused by an inward crushing of an outer sleeve at annular places between the beads.
A fourth type of prior art is exemplified by U.S. Pat. No. 2,779,279, wherein an explosive force is utilized to expand portions of the tubing into an outer coupling sleeve.
A fifth type of prior art is exemplified by U.S. Pat. No. 1,961,453, which was stated to be an improvement on the first-mentioned prior art form. In this patent, the conical shape of the prior art separate sleeve was replicated on the tube wall itself. This, again, gives a large radially inward crushing force when the fitting is assembled.
The first-mentioned prior art patent has the disadvantage that the separate parts are easy to be misplaced and lost, the production variation in the size of the tubes means that the sleeve often fits either tightly or loosely, and if the coupling is overtorqued, the sleeve can dig into the tubing sufficiently that the tube will break easily upon vibration. Also, it is so deformed once tightened that it cannot be taken off and used on another tube.
The fifth-mentioned prior art patent has the disadvantage that the high radially inward force still tends to crush the tube when the fitting is assembled, and even with a double-walled tube, this tendency is still present.